1. Field of the Invention
The present invention relates to multiphase alternating current (AC) motor driving devices for driving a multiphase AC motor with an inverter circuit, in particular, to a multiphase AC motor driving device for determining the occurrence of failure based on the detection results of the current values of the respective phases.
2. Description of the Related Art
An electric power steering device in a vehicle is provided with an AC motor such as a three-phase brushless motor in order to apply to a steering mechanism a steering aiding force corresponding to a steering torque of a handle. A motor drive unit includes a PWM (Pulse Width Modulation) circuit for generating PWM signals having a duty ratio corresponding to the command value and an inverter circuit including pairs of upper and lower switching devices, which are turned ON/OFF according to the duty ratio of the PWM signal output from the PWM circuit, provided for respective phases. The inverter circuit outputs voltage for the respective phases corresponding to the aforementioned duty ratio based on the ON/OFF operations of the switching devices, to drive the motor with the voltages. The currents in the respective phases of the motor are detected by a current detection resistance connected in series with the switching device, and the detected value is the value of the current actually flowing through the motor.
Japanese Laid-Open Patent Publication No. 2005-143153, No. H6-253585 to No. 2007-189825 described below disclose a multiphase AC motor driving device using the inverter circuit. In the device of Japanese Laid-Open Patent Publication No. 2005-143153, an estimated value of the current flowing through the inverter circuit is calculated based on an application voltage and a resistance value between a power supply and a motor, and an actual current value detected in an actual current detecting section and the estimated current value are compared to detect abnormality in the values of the currents flowing through the respective phases. In the device of Japanese Laid-Open Patent Publication No. H6-253585, using the fact that the sum of the current values of the respective phases of the three-phase AC motor is zero, determination is made as abnormal when the sum of the detected current values of the respective phases is not zero. In the device of Japanese Laid-Open Patent Publication No. 2007-189825, the phase currents flowing through the reflux diodes connected in parallel to the switching devices are detected during a dead time period in which the upper and lower switching devices of the inverter circuit are both turned OFF to lengthen the sampling period and enable motor control with high accuracy.
In the motor driving device using the inverter circuit as described above, the sum of the currents of the respective phases of the motor is known to be zero, and thus the occurrence of failure of the motor driving device or the motor can be determined by detecting the currents of the respective phases and determining whether the sum is zero as in Japanese Laid-Open Patent Publication No. H6-253585.
However, if the motor rotates by external force when all the switching devices of the inverter circuit are turned OFF, that is, when the motor driving device is not driving the motor, induction voltage generates in the motor and currents flow through the inverter circuit. As a result, the sum of the current values of the respective phases detected by the current detection resistances does not become zero, and erroneous determination that failure has occurred is made. This will be specifically described in FIG. 9.
FIG. 9 shows a motor driving device shown in FIG. 1 of Japanese Laid-Open Patent Publication No. 2007-189825. The motor driving device includes a control portion 1, a PWM circuit 2, an inverter circuit 3, sample-and-hold circuits 5u to 5w, and direct current (DC) amplification circuits 6u to 6w. The inverter circuit 3 includes pairs of upper and lower arms provided in association with respective phases, the respective arms being configured by switching devices Q1 to Q6, and reflux diodes D1 to D6 connected in parallel thereto. A PWM signal is individually provided from the PWM circuit 2 to the respective gates of the switching devices Q1 to Q6, and the switching devices Q1 to Q6 are turned ON in an ON zone of the PWM signal and are turned OFF in an OFF zone of the PWM signal. Through the ON/OFF operation of the switching devices Q1 to Q6, a U-phase voltage, a V-phase voltage, and a W-phase voltage for the motor drive are extracted from the connection points a, c, and e of the upper and lower arms of the respective phase and are supplied to the three-phase AC motor 4.
Ru, Rv, and Rw are current detection resistances for detecting the phase current of the motor 41 and the voltage generated across the opposite ends of each resistance Ru to Rw (electric potential at points b, d, f) is input to the respective sample-and-hold circuit 5u to 5w. The sample-and-hold circuits 5u to 5w include switches Su to Sw, condensers Cu to Cw, and differential amplifiers Au to Aw. The sampled and held voltage is amplified in the DC amplification circuits 6u to 6w, and output as phase currents Iu to Iw. The control portion 1 is input with a torque value, a vehicle speed value, and a phase current detection value, and outputs a command voltage and a sampling signal. The command voltages Vu to Vw are supplied to the PWM circuit 2, and the sampling signals SPu to SPw are provided to the sample-and-hold circuits 5u to 5w. 
In the circuit of FIG. 9, the switching devices Q1 to Q6 are turned OFF if the motor 4 is not driven by the inverter circuit 3. If external force is applied to the motor 4 in this state, the motor 4 rotates by the external force and induction voltage is generated at windings 4u to 4w of the motor 4. Due to such induction voltage, current indicated by a broken line in the figure flows through the inverter circuit 3 in the direction of the arrow. As a result, current, which originally does not flow, flows through the current detection resistances Rv and Rw, and the V-phase current Iv and the W-phase current Iw of the motor 4 are detected through the sample-and-hold circuits 5v, 5w and the DC amplification circuits 6v, 6w. Thus the sum Iu+Iv+Iw of the current values of the respective phases does not become zero, and the control portion 1 determines that failure has occurred. The sum of the currents does not become zero because external force is applied on the motor 4 and induction voltage is generated, and not because abnormality is found in the motor 4 and the drive circuit, and thus the determination is an erroneous determination.
A case where all the switching devices Q1 to Q6 are turned OFF includes a case where the inverter circuit 3 starts activation, a case where all the PWM signals are Low during the drive of the inverter circuit 3, and the like. The former is when all the switching devices Q1 to Q6 are turned OFF while the ignition switch of the vehicle is turned ON and initial diagnosis operation is being performed. The latter is a case when all the switching devices Q1 to Q6 are turned OFF as a result of having all the PWM signals to Low to prevent destruction of the element when the application voltage of the switching devices Q1 to Q6 exceeds a predetermined value during the operation of the inverter circuit 3.
All the switching devices Q1, Q3, Q5 on the upper side are turned ON and all the switching devices Q2, Q4, Q6 on the lower side are turned OFF to apply breaking at a maximum force when applying a regenerative breaking on the motor 4 while the inverter circuit 3 is operating, but since the currents flow through the current detection resistances due to the generated induction voltage if external force is applied on the motor 4 even when only the switching devices Q2, Q4, Q6 on the lower side are all turned OFF, the problem of erroneous determination described above arises.
In the conventional device, when induction voltage is generated in the motor in a state all the switching devices on the lower side of the inverter circuit are at least turned OFF, that is, in a state the phase currents of the motor cannot be normally detected, determination is erroneously made that failure has occurred when actually failure has not occurred.
It is an object of the present invention to provide a multiphase AC motor driving device which does not make an erroneous determination on the occurrence of failure.